Field of the Invention
The invention relates to an apparatus and a method for the after-treatment of exhaust gases originating from an internal combustion engine operating with excess air. The apparatus has an exhaust-gas after-treatment system with a reduction catalytic converter for reducing NOx components in the exhaust gas, an electrically driven metering device for the controlled introduction of reducing agent into an exhaust-gas line, upstream of the reduction catalytic converter, and a device for cooling the metering device. In the corresponding method, NO.sub.x components in the exhaust gas are reduced by means of the reduction catalytic converter, reducing agent is introduced into an exhaust-gas line upstream of the reduction catalytic converter, by the electrically driven metering device, and the metering device is cooled by means of a device.
Diesel internal combustion engines and lean-burn spark-ignition internal combustion engines, in which fuel is injected directly into the combustion chamber in the cylinder, have a tendency toward high NO.sub.x emissions because of the high excess of oxygen with which they are operated. The so-called selective catalytic reduction (SCR) method is known for reducing the NO.sub.x content in the exhaust gas from such internal combustion engines. Here, a reducing agent is introduced into the exhaust-gas stream at a point upstream of a reduction catalytic converter, in the exhaust gas flow direction, with the aid of an electrically driven metering device, and converts the nitrogen oxides contained in the exhaust gas into harmless nitrogen N.sub.2 and water H.sub.2 O in the presence of oxygen at the reduction catalytic converter.
Such a method is described, for example, by Schoppe et al. in the publication "Ein geregeltes Abgasnachbehandlungssystem zur Erfullung zukunftiger Emissionsgrenzwerte bei PKW-Diesel-motoren" [A Controlled Exhaust Gas Aftertreatment System to Fulfill Future Emission Limits for Passenger Car Diesel Engines], 17th International Vienna Motor Symposium, 1996, Vol. 1. The reducing agent used can be ammonia NH.sub.3, but for reasons of its ability to be handled, an aqueous solution of urea is normally used. However, it is also possible to use fuel or derivatives as a reducing agent.
In such exhaust-gas after-treatment systems, a control device of the internal combustion engine, or a separate control device, often referred to as a DeNO.sub.x control device, continuously calculates the intended quantity of reducing agent to be metered on the basis of operating parameters of the internal combustion engine, such as the quantity of fuel introduced into the combustion chamber, the operating temperature and the rotational speed, as well as the temperature of the reduction catalytic converter, for example.
The metering device used in this case is preferably a conventional injection valve, such as is used, for example, in a low-pressure gasoline injection system. In accordance with the changed intended use of such an injection valve, namely of injecting reducing agent, for example urea, at least those components of the valve which are directly exposed to the urea are produced from urea-resistant materials (noble metal, coated plastic), and the geometry of the outlet opening of the valve is adapted to the quantities to be ejected, which are smaller than when metering fuel.
The use of conventional low-pressure gasoline injection valves is generally permissible only up to a specific temperature, typically about 130.degree. C. The reason for this lies in the coil construction, in particular the insulation of the coils for the electromagnetic drive and the thermal resistances of the materials used, for example of the seals.
Since the injection valve is arranged directly on the exhaust-gas pipe, upstream of the reduction catalytic converter, this maximum temperature is almost always reached outside the warming-up phase of the internal combustion engine.
However, if the injection valve or at least parts thereof are heated above this maximum permissible temperature for a relatively long time, then functional disruption occurs, up to the complete failure of the injection valve.
In addition, an excessively high temperature at the injection valve has negative influences on the reducing agent to be introduced. In the case where aqueous urea solution is used as the reducing agent, the solution must not be heated above specific temperature limits, since otherwise the aqueous urea solution begins to decompose thermally or to crystallize out.
In order to solve this thermal problem, it is possible to provide a relatively long line connection between the metering point of the reducing agent and the point of entry into the exhaust gas. Here, however, there is the risk that the urea, which is already finely distributed, will recombine again to form large droplets, and that an irregular metering rate will occur.
Another possibility for cooling the injection valve, in the case of a so-called bottom-fed injection valve, is to flush part of the valve body continuously with an aqueous urea solution and in this way to carry away the heat. This assumes that there is a continuous urea circuit from the urea tank to the injection valve and back which, of course, entails a considerable constructional outlay.
In accordance with U.S. Pat. No. 5,605,042 (German published patent application DE 44 36 397 A1), reducing agent is introduced into the exhaust-gas fed to the reduction catalytic converter with an electrically controlled metering valve that is combined with a control valve in a common housing. The control valve is used for the controlled introduction of supplied pressurized air, in which a stored quantity of reducing agent processed via the metering valve is added intermittently to the exhaust gas. The control valve and the metering valve are arranged in a common supporting body, around which there flows cooling water from the coolant loop of the internal combustion engine. Although, by means of such a configuration, the maximum temperature at the end of the injection-valve holder can be limited to the coolant temperature (max. 90.degree.-100.degree. C.) and the temperature stress on the urea circuit can be relieved, a cooling jacket, lines and hose connections are additionally needed.